Standard of care for patients with deep venous thrombosis (blood clot in the leg or arm) and the complication of pulmonary embolus (blood clot in the lung) typically includes anti coagulation (blood thinning) medication for a 3 to 6 month period with full dosewarfarin (coumadin). Patients following this regimen experience high rates of recurrent thrombosis, and re-hospitalization in the years immediately following cessation of anticoagulation. This is true for patients at average risk of disease recurrence, and for patients at increased risk due to the presence of inherited defects of anticoagulation such as the factor V Leiden mutation. No clinical regimen currently available has proven to have sufficient benefit to support long term prophylaxis. Simply put, using current standards of anticoagulation with warfarin in the long term, the undoubted benefit in preventing clots is often negated by the excess risk of bleeding. In contrast, while targeted low dose warfarin is safe for long term therapy with respect to bleeding risk, the effectiveness of this approach is untested. The research question is: Will long term low dose warfarin effectively prevent venous thromboembolic disease? The Prevention of Recurrent Venous Thromboembolism (PREVENT) trial will evaluate the effectiveness of long term therapy with low dose warfarin in the secondary prevention of venous thromboembolism (VTE). Patients with a history of documented unexplained venous thrombosis who have completed a standard course of anticoagulation therapy will be enrolled in a randomized, double-blind, placebo-controlled trial comparing usual care plus low-dose warfarin to usual care plus placebo for a period of up to 4years. Trial end-points will include recurrent VTE, major bleeding episodes, and all deaths in the total patient population and separately in those patients with factor V Leiden. The potential clinical impact of the PREVENT trial is broad since a positive finding would strongly support chronic low dose anticoagulation among patients with venous thrmobosis who are at risk for recurrence following cessation of standard outpatient anticoagulation. This intervention trial will be conducted among 800 men and women ages 40 and over who have had a prior venous thrombosis, and thus may be at risk for future recurrence. With 40 centers nationwide participating in this trial we estimate approximately 30 patients will be recruited from Fairview-University Medical Center (F-UMC). Participation will involve 3-4 years of study treatment and follow-up. The General Clinical Research Center (GCRC) will provide physical space to facilitate all patient visits at each time point. GCRC nursing and ancillary staff will be asked to obtain applicable measurements, vital signs, perform venipuncture, necessary centrifuging of blood speciments, and point of care unblinded Coag-u-check testing for INR (see physician orders). In addition, nursing staff will be asked to document current medications at all time points. An in-service will be provided to nursing staff who will need to perform Coag-u-check INR. Although 29 visits are expected per patient there may be accasional unscheduled visits.